1. Field of the Invention
This invention relates to solid state camera apparatus which performs digital signal processing on the output signal of a solid state image-sensing device so as to form an output video signal.
2. Description of the Prior Art
In general, in a solid state camera apparatus using a solid state image sensor having a discrete pixel structure, such as a CCD image sensor, since the solid state image sensor itself is a sampling system, it is known that an unwanted (loopback) component from the spatial sampling frequency f.sub.CCD is mixed in the image signal of the solid state sensor. In the past, an optical system has been provided, including a birefringence-type optical low-pass filter which suppresses the high frequency component of the baseband component of the image signal so as to satisfy the Nyquist conditions of the sampling system of the solid state image sensor, and prevent the addition of the unwanted component to the baseband component of the image signal.
In color television camera apparatuses which pick up color images, two-plate solid state apparatuses which pick up three primary-color images using a solid state image sensor for picking up green color images and solid state image sensors provided with color coding filters for red color pixels and blue color pixels, and three-plate type solid state apparatuses which pick up three primary-color images by separate solid state image sensors, and other multiple-plate solid state apparatuses are being commercialised.
Moreover, as a means for achieving an improvement in the resolution of the above multiple-plate type solid state apparatus, a so-called spatial pixel shifting method is known wherein image sensors for picking up red color images and for picking up blue color images are shifted by exactly 1/2 of the spatial sampling period of the pixels with respect to the solid state image sensor for picking up the green images. By using the spatial pixel shifting method, it is possible to realize a high resolution exceeding the limits of the number of pixels of the solid state image sensor in an analog output multiple-plate type solid state apparatus.
Also, as a standard for digital video tape recorders for commercial use in broadcasting stations etc, there are the D-1 standard, the D-2 standard, etc. Digital interfaces for the digital video related equipment meeting these standards are required for color television camera apparatus as well.
In the D-1 standard, which is the standard for 4:2:2 digital component video signals, the sampling frequency is 13.5 MHz, which corresponds to 858 times the horizontal frequency (f.sub.H(NTSC)) in the NTSC system, and corresponds to 864 times the horizontal frequency (f.sub.H(PAL)) in the
system. Locking is possible by a whole multiple of the horizontal frequencies in both systems. In the D-2 standard, which is a standard for digital component video signals, the sampling frequency is made 4f.sub.sc, that is four times the sub-carrier frequency, which minimizes the beat interference of the sub-carrier and the sampling clock. The sampling frequency f.sub.S(NTSC) in the NTSC system is 14.3 MHz and the sampling frequency f.sub.S(PAL) in the PAL system is 17.73 MHz.
As mentioned above, however, when trying to realize a color television camera apparatus which directly outputs a digital image signal complying with the D-1 standard or the D-2 standard, directly to output a digital video signal with a high resolution, little loopback strain, and an excellent picture quality, there is the problem that one can only obtain a partially-effective optical low-pass filter to be used for the prefilter for the solid state image sensor, that is, one with a sloping cut-off characteristic. Therefore the sampling rate (number of pixels) of the solid state image sensor used for tlie camera unit must be made higher than the sampling rate in the D-1 standard and the D-2 standard, if one considers the difficulty in obtaining both more excellent MTF characteristics and a reduced loopback strain component. Moreover, for image signals obtained by the solid state image sensor, if one considers the performance of the defect correction processing for each pixel of the solid state image sensor, etc, by digital processing, the prevention of the occurrence of beat interference, etc, it is desirable to make the sampling rate of the solid state image sensor and the sampling rate in the analog/digital conversion means for digitizing the image signals obtained by the solid state image sensor match. Also, as mentioned above, in the units of color television camera apparatuses which directly output digital image signals complying with the D-1 standard or D-2 standard, use has been made of solid state image sensors exclusively designed for these standards, with numbers of pixels individually set to correspond to the D-1 standard or D-2 standard.